[SPARK-19762][ML][FOLLOWUP] Add necessary comments to L2Regularization.

## What changes were proposed in this pull request?
MLlib ```LinearRegression/LogisticRegression/LinearSVC``` always standardize the data during training to improve the rate of convergence regardless of _standardization_ is true or false. If _standardization_ is false, we perform reverse standardization by penalizing each component differently to get effectively the same objective function when the training dataset is not standardized. We should keep these comments in the code to let developers understand how we handle it correctly.

## How was this patch tested?
Existing tests, only adding some comments in code.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #18992 from yanboliang/SPARK-19762.